Tub Surround Remodel Question/Concerns

Hello all. I'm new to the forums and would like to get some expert advise on a possible issue with my tub surround re-do. I have a 70s home with a 30"/60"/30" 3 wall fiberglass surround and tub. I gutted the bathroom so I could update it. I plan on tiling instead of another 3 piece surround and my issue is this. I removed the drywall from the tub 3 walls so I could replace it with Hardibacker and Kerdi membrane. Under the drywall on the 30" wall opposite the wet wall, I found a layer of Celotex. The rough opening is perfect if I leave the Celotex in and 1/2" Hardibacker over it. I could see how this would not be an issue with a solid surface surround. Since I am using Kerdi membrane, my concern is the Celotex in the wall causing a vapor trap. I know Kerdi is suppose to be 100% waterproof if installed correctly but you are not suppose to ad the vapor barrier underneath the Hardibacker when using Kerdi due to the vapor trap issue. My question is ...will the Celotex act as a vapor barrier inside the wall causing the issues I fear? If I remove it I have to make up the 1/2-3/4" gap, plus shim further to clear the tub lip for tile. Also the next sheet of drywall outside the tub area will not be level with the Hardibacker without shimming the whole wall out. I was able to remove the tub drywall and the next piece perfectly so I could drop in Hardibacker for a perfect, level fit. Should I be concerned by leaving in the Celotex? This is the wall that will get the most wet with the addition of the shower. Thanks in advance!